Deposition process for solar cell front contact

ABSTRACT

A method includes depositing an acid over a portion of a buffer layer of a solar cell substrate. A front contact material is deposited over the buffer layer, such that the front contact material does not bond to the portion of the buffer layer having the acid on it. Thus, the front contacts of adjacent solar cells of the solar cell substrate are formed with a separation between them.

PRIORITY CLAIM AND CROSS-REFERENCE

None.

BACKGROUND

This disclosure related to fabrication of thin film photovoltaic cells.

Solar cells are electrical devices for generation of electrical currentfrom sunlight by the photovoltaic (PV) effect. Thin film solar cellshave one or more layers of thin films of PV materials deposited on asubstrate. The film thickness of the PV materials can be on the order ofnanometers or micrometers.

Examples of thin film PV materials used as absorber layers in solarcells include copper indium gallium selenide (CIGS) and cadmiumtelluride. Absorber layers absorb light for conversion into electricalcurrent. Solar cells also include front and back contact layers toassist in light trapping and photo-current extraction and to provideelectrical contacts for the solar cell. The front contact typicallycomprises a transparent conductive oxide (TCO) layer. The TCO layertransmits light through to the absorber layer and conducts current inthe plane of the TCO layer. In some systems, a plurality of solar cellsare arranged adjacent to each other, with the front contact of eachsolar cell conducting current to the next adjacent solar cell. Eachsolar cell includes an interconnect structure for conveying chargecarriers from the front contact of a solar cell to the back contact ofthe next adjacent solar cell on the same panel. The interconnectstructure reduces the area available for photon collection.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1A is a plan view of a solar cell substrate, in accordance withsome embodiments.

FIG. 1B is a cross sectional view of the solar cell substrate of FIG.1A, in accordance with some embodiments.

FIG. 2A is a plan view of the solar cell substrate of FIG. 1B with anacid line formed thereon, in accordance with some embodiments.

FIG. 2B is a cross sectional view of the solar cell substrate of FIG.2A, in accordance with some embodiments.

FIG. 3A is a plan view of the solar cell substrate of FIG. 2B with thefront contact formed thereon, in accordance with some embodiments.

FIG. 3B is a cross sectional view of the solar cell substrate of FIG.3A, in accordance with some embodiments.

FIG. 4 is a flow chart of a method in accordance with some embodiments.

FIGS. 5A to 5C show examples of methods for performing step 410 of FIG.4, in accordance with some embodiments.

FIG. 6A is a scanning electron microscope image of a transparentconductive oxide (TCO) material of a substrate in accordance with someembodiments.

FIG. 6B is a scanning electron microscope image of exposed absorbermaterial on the substrate of FIG. 6A, in a region where TCO bonding isprevented by depositing acid on the region.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the subject matter.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In this disclosure and the accompanying drawings, like referencenumerals indicate like items, unless expressly stated to the contrary.

Some embodiments described herein provide methods of forming a P3 linewhich separates front contacts of adjacent solar cells within the samesolar panel. The methods use deposition steps without mechanicalscribing. In some embodiments, the front contact is formed by selectivechemical vapor deposition (CVD) to form the P3 line.

FIGS. 3A and 3B show the solar panel 100 as it is configured after frontcontact formation, in accordance with some embodiments. The portion ofthe solar panel 100 shown in FIGS. 3A and 3B includes an interconnectstructure 172, which provides a series connection between two adjacentsolar cells of the panel 100. In FIGS. 3A and 3B, the width of theinterconnect structure 172 is exaggerated relative to the width of thecollection region 170 for clarity, but the collection region 170 isactually much wider than the interconnect structure 172.

The solar cell 100 includes a solar cell substrate 110, a back contactlayer 120, an absorber layer 130, a buffer layer 140 and a front contactlayer 150.

Substrate 110 can include any suitable substrate material, such asglass. In some embodiments, substrate 110 includes a glass substrate,such as soda lime glass, or a flexible metal foil or polymer (e.g., apolyimide, polyethylene terephthalate (PET), polyethylene naphthalene(PEN)). Other embodiments include still other substrate materials.

Back contact layer 120 includes any suitable back contact material, suchas metal. In some embodiments, back contact layer 120 can includemolybdenum (Mo), platinum (Pt), gold (Au), silver (Ag), nickel (Ni), orcopper (Cu). Other embodiments include still other back contactmaterials. In some embodiments, the back contact layer 120 is from about50 nm to about 2 μm thick.

In some embodiments, absorber layer 130 includes any suitable absorbermaterial, such as a p-type semiconductor. In some embodiments, theabsorber layer 130 can include a chalcopyrite-based material comprising,for example, Cu(In,Ga)Se₂ (CIGS), cadmium telluride (CdTe), CulnSe₂(CIS), CuGaSe₂ (CGS), Cu(In,Ga)Se₂ (CIGS), Cu(In,Ga)(Se,S)₂ (CIGSS),CdTe or amorphous silicon. Other embodiments include still otherabsorber materials. In some embodiments, the absorber layer 140 is fromabout 0.3 μm to about 8 μm thick.

Buffer layer 140 includes any suitable buffer material, such as n-typesemiconductors. In some embodiments, buffer layer 140 can includecadmium sulphide (CdS), zinc sulphide (ZnS), zinc selenide (ZnSe),indium(III) sulfide (In₂S₃), indium selenide (In₂Se₃), orZn_(1-x)Mg_(x)O, (e.g., ZnO). Other embodiments include still otherbuffer materials. In some embodiments, the buffer layer 140 is fromabout 1 nm to about 500 nm thick.

In some embodiments, front contact layer 150 includes an annealedtransparent conductive oxide (TCO) layer of constant thickness of about100 nm or greater. The terms “front contact” and “TCO layer” are usedinterchangeably herein; the former term referring to the function of thelayer 150, and the latter term referring to its composition. In someembodiments, the charge carrier density of the TCO layer 150 can be fromabout 1×10¹⁷ cm⁻³ to about 1×10¹⁸ cm⁻³. The TCO material for theannealed TCO layer can include suitable front contact materials, such asmetal oxides and metal oxide precursors. In some embodiments, the TCOmaterial can include AZO, GZO, AGZO, BZO or the like) AZO: alumina dopedZnO; GZO: gallium doped ZnO; AGZO: alumina and gallium co-doped ZnO;BZO: boron doped ZnO. In other embodiments, the TCO material can becadmium oxide (CdO), indium oxide (In₂O₃), tin dioxide (SnO₂), tantalumpentoxide (Ta₂O₅), gallium indium oxide (GaInO₃), (CdSb₂O₃), or indiumoxide (ITO). The TCO material can also be doped with a suitable dopant.

In some embodiments, in the doped TCO layer 150, SnO₂ can be doped withantimony, (Sb), flourine (F), arsenic (As), niobium (Nb) or tantalum(Ta). In some embodiments, ZnO can be doped with any of aluminum (Al),gallium (Ga), boron (B), indium (In), yttrium (Y), scandium (Sc),fluorine (F), vanadium (V), silicon (Si), germanium (Ge), titanium (Ti),zirconium (Zr), hafnium (Hf), magnesium (Mg), arsenic (As), or hydrogen(H). In other embodiments, SnO₂ can be doped with antimony (Sb), F, As,niobium (Nb), or tantalum (Ta). In other embodiments, In₂O₃ can be dopedwith tin (Sn), Mo, Ta, tungsten (W), Zr, F, Ge, Nb, Hf, or Mg. In otherembodiments, CdO can be doped with In or Sn. In other embodiments,GaInO₃ can be doped with Sn or Ge. In other embodiments, CdSb₂O₃ can bedoped with Y. In other embodiments, ITO can be doped with Sn. Otherembodiments include still other TCO materials and corresponding dopants.

The layers 120, 130, 140 and 150 are provided in the collection regions170. Solar cell 100 also includes an interconnect structure 172 thatincludes three lines, referred to as P1, P2, and P3. The P1 scribe lineextends through the back contact layer 130 and is filled with theabsorber layer material. The P2 scribe line extends through the bufferlayer 140 and the absorber layer 130, and contacts the back contact 120of the next adjacent solar cell. The P3 line extends through the frontcontact layer 150, but not the buffer layer 140 or absorber layer 130.The P3 line of the adjacent solar cell is immediately to the left of thesolar cell collection region 170.

The P3 line separates the front contacts 150 of adjacent solar cells, sothat each front contact can transmit current through the P2 scribe lineto the back contact of the next adjacent solar cell without shortingbetween front adjacent contacts. The front contact layer 150 has arespective P3 line (separation region) in each solar cell, in which theabsorber layer 130 and buffer layer 140 are continuous beneath the P3separation region, but no front contact (TCO) material is present in theseparation region. In the configuration of FIGS. 3A and 3B, the absorberlayer 130 and buffer layer 140 are formed in a region 160 below the P3line. This provides additional photon collection area, reducing thenon-collecting “dead zone” in the interconnect structure 172. Chargecarriers generated at the p-n junction within the region 160 flow to theadjacent collection region 170 (to the right in FIG. 3B) and arecollected by the front contact of the adjacent cell.

In some embodiments, the P3 separation region has a width W smaller than100 micrometers. In some embodiments, the P3 separation region has awidth W of about 70 micrometers. This width is about 100 micrometerssmaller than a corresponding width of a P3 scribe line achieved bymechanical scribing. Because a solar panel can include about 100 solarcells (each with a respective P3 line), the total savings in P3 linewidth is about 100×100 μm=10,000 μm=1 cm. This corresponds to anincrease of 1 cm in the length of the collection area, or an increase of55 cm² for a solar panel having 100 solar cells with a panel width of 55cm.

Also, because the front contact 150 is formed by deposition processeswithout and material removal step, the TCO material has an edge 152without cracks on each side of the separation region. TCO materialremoval methods, such as mechanical scribing can cause cracks in the TCOmaterial, but the front contact layer 150 described herein is free ofcracks.

Further, because there is no concern about crack formation during P3line formation, the P3 line can be located closer to the P2 scribe linewithout risk of a crack adjacent the P3 line propagating to the edge ofthe P2 line. Thus, additional reduction in the width of the interconnectstructure 172 can be achieved.

FIG. 4 is a flow chart of a method of forming the solar cell of FIGS. 3Ato 3B. FIGS. 1A to 3B show steps in the formation of a solar panel 100.

At step 402, the back contact 120 is formed over the solar cellsubstrate 110. The back contact can deposited by PVD, for examplesputtering, of a metal such as Mo, Cu or Ni over the substrate, or byCVD or ALD or other suitable techniques.

At step 404, the P1 scribe line is formed through the back contact layer120. For example, the scribe line can be formed by mechanical scribing,or by a laser or other suitable scribing process. Each solar cell in thepanel 100 has a respective P1 scribe line.

At step 406, an absorber layer 130 is formed over the back contact layer120. The absorber layer 130 can be deposited by PVD (e.g., sputtering),CVD, ALD, electro deposition or other suitable techniques. For example,a CIGS absorber layer can be formed by sputtering a metal filmcomprising copper, indium and gallium then applying a selenizationprocess to the metal film.

At step 408, the buffer layer 140 is formed over the absorber layer 130.The buffer layer 140 can be deposited by chemical deposition (e.g.,chemical bath deposition), PVD, ALD, or other suitable techniques.

At step 410, the P2 scribe line is formed and a P3 deposition isperformed without scribing the P3 line. This step is discussed below inthe description of FIGS. 5A to 5C. The configuration of the substrate atthe conclusion of this step is shown in FIGS. 2A and 2B.

At step 412, the front contact layer 150 is formed over the buffer layer140, which is over the absorber layer 130. This step includes depositinga front contact material (TCO) over the buffer layer 140, such that thefront contact material does not bond to the portion of the buffer layerhaving the acid 142 thereon, thereby forming front contacts of adjacentsolar cells of the solar cell substrate with a separation therebetween.

In some embodiments, the step of depositing the front contact materialcomprises chemical vapor deposition (CVD), such as metal organicchemical vapor deposition (MOCVD). In other embodiments, the frontcontact material is deposited by low pressure chemical vapor deposition(LPCVD) or by plasma enhanced chemical vapor deposition (PECVD).

The front contact material (TCO) is deposited over the buffer layer,such that the front contact material does not bond to the portion of thebuffer layer 140 having the acid 142 thereon. Front contacts of adjacentsolar cells of the solar cell substrate are thus formed with aseparation between them, without requiring any mechanical scribing.

In some embodiments, no P3 material removal step is performed. In someembodiments, following the TCO deposition, the acid solution evaporatesfrom the P3 line without requiring any cleaning step. In someembodiments, where an additive (such as silicon oxide particles) isincluded in the acid, the silicon acid can remain in the P3 line afterTCO deposition. Thus, in some embodiments, additives in the acidsolution 142 can be volatile, while in other embodiments, the additivescan be transparent and non-conductive, and can be allowed to remain inthe P3 line after front contact formation. A transparent, non-conductivematerial will not interfere with photon collection, nor form a bridgebetween adjacent front contacts 150. Thus, allowing a transparent,non-conductive additive to remain in the P3 line after front contactformation does not interfere with solar panel performance or efficiency.

FIG. 5A shows a method 410A of performing step 410, in accordance withsome embodiments.

The method 410A includes sequential formation of the P2 scribe line, andP3 deposition.

At step 502, the P2 scribe line can be formed by mechanical scribing, orby a laser or other suitable scribing process. The configuration of thesubstrate at the conclusion of this step is shown in FIGS. 1A and 1B.

At step 504, an acid 142 is deposited over a portion of the buffer layer140. In some embodiments, the step of depositing the acid 142 includesprinting the acid on the buffer layer using a printing head of ascribing tool. The printing head is one of a number of commerciallyavailable devices which can be mounted behind the mechanical tip of thescribing tool. In this example, the printing step 504 is performedsequentially after the P2 scribing step 502. In other embodiments, theP2 scribing step is performed sequentially after the printing.

The acid 142 can be any acid solution which prevents TCO deposition orbonding between the TCO material and the underlying absorber or buffermaterial, but does not etch the underlying absorber or buffer material.In some embodiments, the acid solution is a volatile liquid, so thatfollowing TCO deposition, any remaining acid evaporates withoutrequiring any special cleaning process. In some embodiments, the acidcomprises HCl or H₂SO₄. For example, in some embodiments, the absorberlayer is CIGS, the buffer layer 140 comprises ZnO, and the acid is asolution of HCl in water, with a concentration of the HCl in a rangefrom about 0.2 mol to about 1.0 mol. In other embodiments, an HClsolution is used to prevent deposition of an SnO TCO material on a ZnObuffer layer. An appropriate acid solution can be selected for any othercombination of buffer layer material and TCO material.

In some embodiments, the acid 142 further comprises an additive forcontrolling spreading of the acid, for example, by controlling thesurface tension of the solution. For example, in some embodiments, theadditive comprises silicon oxide particles. The additive prevents theline of acid 142 from spreading and increasing the width W of the P3line.

FIG. 5B show a variation of the acid depositing process 410B, whereinthe printing is performed while a P2 scribe line is being mechanicallyscribed in the solar cell substrate, where the P2 scribe line penetratesthe buffer layer and absorber layer of the solar cell substrate.

At step 512, the P2 scribe line is scribed.

At step 514, the acid is deposited simultaneously by printing on thebuffer layer. The acid solution can be the same as described above forthe example of FIG. 5A. The scribing tool is configured to scribe the P2line and, at the same time, print a line of the acid solution. Becausestep 514 deposits the acid at the same time as the existing P2 scribingprocess, the total process time that would be spent performing P3scribing is eliminated. For a solar panel having about 100 P3 lines,each about 55 cm in length, this results in a reduction in total processtime (for fabricating a solar panel) of about 50 seconds.

FIG. 5C shows another example of a process for forming the P2 scribeline and the P3 deposition.

At step 522, the P2 scribe line is formed by mechanical scribing, or bya laser or other suitable scribing process. The configuration of thesubstrate at the conclusion of this step is shown in FIGS. 1A and 1B.

At step 522, the P3 line formed using a mask (not shown). For example, amask can be placed over the solar cell substrate, where the mask hasopenings in the form of lines corresponding to the P3 lines. The theacid 142 can be sprayed over the entire mask, but is only deposited onthe buffer layer 140 in the P3 regions. In some embodiments, a singlenozzle applies the spray and scans along the length of each P3 line,sequentially. In other embodiments, a plurality of nozzles are arrangedin a line, for spraying the acid along an entire P3 line, so eachindividual P3 line can be sprayed along its length all at once. In otherembodiments, a two dimensional array of nozzles is provided, forspraying the entire solar panel simultaneously.

The configuration of the substrate at the conclusion of any of theprocesses 410A, 410B or 410C is as shown in FIGS. 2A and 2B.

FIGS. 6A and 6B are scanning electron microscope images taken from twoportions of a substrate. FIG. 6A shows the crystalline structure of aZnO TCO layer. FIG. 6B shows the crystalline structure of an exposedabsorber layer material. The substrate was processed by depositing asolution of HCl in water on the portion of the substrate shown in FIG.6B, and then subjecting the entire substrate to the MOCVD gas. Theregion shown in FIG. 6B has larger rougher crystals indicating theabsorber material, whereas the region on which the TCO bonded to thebuffer layer (as shown in FIG. 6A) has smaller, more triangularcrystals.

The selective deposition process described herein can be used not onlyfor the P3 line, but also for any post CVD process pattern. It also canbe used for any display or touch panel post CVD process pattern.

Using the methods describe herein, the P3 line, which separates frontcontacts of adjacent solar cells within the same solar panel, is formedby deposition steps without mechanical scribing. The method eliminates“chipout,” the excess scribe line width that results from mechanicalscribing techniques. A narrower P3 line is provided, increasing theabsorber area available for photon collection, and reducing the size ofthe “dead zone” in the interconnect structure. The resulting frontcontacts have edges adjacent the P3 line, which are free from cracksbecause the P3 line is formed without mechanical scribing. Also, in someembodiments, the P3 line can be located closer to the P2 scribe line,reducing the spacing from P1 to P3, thus providing additional reductionin the width of the interconnect structure 172, and additional increasein the area available for photon collection.

In some embodiments, a method comprises: depositing an acid over aportion of a buffer layer of a solar cell substrate; and depositing afront contact material over the buffer layer, such that the frontcontact material does not bond to the portion of the buffer layer havingthe acid thereon, thereby forming front contacts of adjacent solar cellsof the solar cell substrate with a separation therebetween.

In some embodiments, a method comprises: forming a back contact over asolar cell substrate; forming an absorber over the back contact; forminga buffer layer over the absorber; depositing an acid over a portion ofthe buffer layer; and depositing a front contact material over thebuffer layer, such that the front contact material does not bond to theportion of the buffer layer having the acid thereon, thereby formingfront contacts of adjacent solar cells of the solar cell substrate witha separation therebetween.

In some embodiments, a solar panel comprises: a solar cell substrate; aback contact over the solar cell substrate; an absorber over the backcontact; a buffer layer over the absorber; and a front contact materialover the buffer layer. The front contact layer has at least oneseparation region in which the absorber layer and buffer layer arecontinuous beneath the separation region, but no front contact materialis present in the separation region. The separation region separatesfront contacts of adjacent solar cells. The separation region has awidth smaller than 100 micrometers.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method comprising: depositing an acid over a portion of a bufferlayer of a solar cell substrate; and depositing a front contact materialover the buffer layer, such that the front contact material does notbond to the portion of the buffer layer having the acid thereon, therebyforming front contacts of adjacent solar cells of the solar cellsubstrate with a separation therebetween.
 2. The method of claim 1,wherein the step of depositing the acid includes printing the acid onthe buffer layer.
 3. The method of claim 2, wherein the printing isperformed using a printing head of a scribing tool.
 4. The method ofclaim 2, wherein the printing is performed while a P2 scribe line isbeing mechanically scribed in the solar cell substrate, the P2 scribeline penetrating the buffer layer and an absorber layer of the solarcell substrate.
 5. The method of claim 1, wherein the separation is a P3line of the solar cell substrate, and the P3 line is formed withoutmechanical scribing.
 6. The method of claim 1, wherein the acid isdeposited using a mask.
 7. The method of claim 1, wherein the step ofdepositing the front contact material comprises chemical vapordeposition.
 8. A method comprising: forming a back contact over a solarcell substrate; forming an absorber over the back contact; forming abuffer layer over the absorber; depositing an acid over a portion of thebuffer layer; and depositing a front contact material over the bufferlayer, such that the front contact material does not bond to the portionof the buffer layer having the acid thereon, thereby forming frontcontacts of adjacent solar cells of the solar cell substrate with aseparation therebetween.
 9. The method of claim 8, wherein the step ofdepositing the acid includes printing the acid on the buffer layer usinga printing head of a scribing tool.
 10. The method of claim 9, whereinthe printing is performed while a P2 scribe line is being mechanicallyscribed in the solar cell substrate, the P2 scribe line penetrating thebuffer layer and absorber layer of the solar cell substrate.
 11. Themethod of claim 10, wherein the separation is a P3 line of the solarcell substrate, and the P3 line is formed without mechanical scribing.12. The method of claim 8, wherein the acid is deposited using a mask.13. The method of claim 8, wherein the step of depositing the frontcontact material comprises metal organic chemical vapor deposition. 14.The method of claim 8, wherein the acid comprises HCl or H₂SO₄.
 15. Themethod of claim 14, wherein the buffer layer comprises ZnO, and the acidis solution of HCl in water, with a concentration of the HCl in a rangefrom about 0.2 mol to about 1.0 mol.
 16. The method of claim 14, whereinthe acid further comprises an additive for controlling spreading of theacid.
 17. The method of claim 16, wherein the additive comprises siliconoxide particles. 18-20. (canceled)
 21. A method comprising: forming aback contact over a solar cell substrate; forming an absorber over theback contact; forming a buffer layer over the absorber; depositing anacid over a portion of the buffer layer; and depositing a front contactmaterial over the buffer layer, such that the front contact materialdoes not bond to the portion of the buffer layer having the acidthereon, thereby forming a separation opening that extends from a topsurface of the front contact material to a top surface of the buffermaterial.
 22. The method of claim 21, wherein the step of depositing theacid includes printing the acid on the buffer layer using a printinghead of a scribing tool.
 23. The method of claim 9, wherein the printingis performed while a P2 scribe line is being mechanically scribed in thesolar cell substrate, the P2 scribe line penetrating the buffer layerand absorber layer of the solar cell substrate.